high speed shadowgraphy of a combusting air blast atomizer spray

ILASS – Europe 2010, 23rd Annual Conference on Liquid Atomization and Spray Systems, Brno, Czech Republic, September 2010

High Speed Shadowgraphy of a Combusting Air Blast Atomizer Spray at ElevatedPressure

C. Willert∗, S. Mößner, S. Freitag and C. HassaInstitute of Propulsion Technology,German Aerospace Center (DLR)

Linder Höhe, 51147 Köln, Germany

AbstractThis contribution demonstrates the use of a compact high repetition rate pulsed high power LED light source incombination with a high speed camera to obtain high speed shadowgraphs of an airblasted spray in a fired researchcombustor at medium pressures. The temporally resolved structures are subjected to conventional spectral analysisand exhibit very dominant fundamental frequencies throughout the visualized spray structures. The image data isfurther compared to time series of drop-sizing measurements made by a phase Doppler instrument. The periodicityis attributed to a spiraling spray release from the nozzle lip that is believed to be induced by a precessing vortex orvortices orbiting around the burner axis.

IntroductionIn gas turbine combustion of liquid fuels, the fuel placement is of primary importance for the ensuing com-

bustion characteristics. Current knowledge shows that the time dependent behavior of the flow as well as that ofthe atomization process of the liquid fuel itself exhibit coherent structures, which can have an influence on macro-scopic features of combustion [1, 2]. Therefore, high speed visualization of the emerging spray and its mixing intothe burner air flow is a useful tool to gain a quick overview on the unsteady phenomena which might be of impor-tance throughout the operation range of the combustor. High speed imaging has seen a very rapid development inrecent years with increasing camera performance at decreasing prices. Yet simple to use, low-maintenance, highspeed light sources remained scarce until the recent introduction of high power LED [3]. In this contribution anapplication of a pulsed LED light source for high speed imaging is exemplified. The temporally resolved structuresof the spray are compared with time series of drop-sizing measurements made by a phase Doppler instrument.

Materials and MethodsThe facility, an optically accessible, single sector combustor, is described in detail in [4]. The general arrange-

ment of the high speed shadowgraphy imaging setup was already described in [5] to study the atomization of asingle kerosene jet issuing into an isothermal cross-flow. For the present application however a new high powerLED source enabled the illumination of a significantly larger field of view allowing the full spray field downstreamof the burner to be imaged at frame rates of 20 kHz. Details of the pulsed LED light source and experimentalprocedures will be presented next.

Figure 1 shows a photograph of the pressurized combustor facility in operation with the pulsed LED lightsource and high speed camera positioned on opposite sides. The components of the in-line illumination arrange-ment are schematically shown in figure 2. The light source is a high power light emitting diode (CBT-120, LuminusDevices) with a peak luminous flux of > 2000 lm at λo = 526 nm and a spectral width of about 40 nm (FWHM).Contrary to other high power LED devices that use a multitude of light emitting elements, this device emits lightuniformly from a single die of 4.6 × 2.6 mm2 area. The diode is driven with pulsed currents of If = 10 A forτp = 2 µs using a laser diode driver (LDP-V 50-100 V3, PicoLAS GmbH). The emitted light is collimated usinga combination of condenser lens (f = 30 mm) and Fresnel lens (f = 100 mm, f# = 1.0) and illuminates theoptical accessible region downstream of the spray nozzle. A high speed CMOS camera (APX-RS, Photron) on theopposite side of the combustor focuses onto this region. Due to the strong flame luminosity (soot radiation) aninterference filter (λo = 532 ± 5 nm) was placed in front of the lens (f = 105 mm, f# = 2.8, Micro Nikkor,Nikon) to reduce the contribution of the flame luminosity and thereby enhancing image contrast.

With the camera frame rate set to 20 kHz the effective image area is reduced to 512 × 256 pixel whichcorresponds to a field of view of 65 mm wide by 32 mm high (magnification ≈ 8 pixel/mm). The camera isoperated in a non-shuttered mode since the stroboscopic illumination by the LED (τp = 2 µs) essentially freezesthe motion of the spray during the 50 µs exposure time of each frame. Image intensity is optimized by adjustingthe drive current of the LED.∗Corresponding author: [email protected]

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ILASS – Europe 2010 High Speed Shadowgraphy of a Combusting Air Blast Atomizer Spray

Figure 1. High speed shadowgraphy set up on the optically accessible single sector combustor facility duringoperation at 4 bar. The LED light source and collimating optics can be seen on the right while the camera islocated on the opposite side to the left of the facility. PDA delivery and receiving optics are also visible

Results and DiscussionThe compact, pulsed LED light source enabled the visualization of the global characteristics of the unsteady

spray in the presence of combustion. The combustor operating conditions are summarized in table 1. Figure 3shows an example of the evolving spray structure obtained at 20 kHz. Here dark regions are associated with highkerosene droplet concentrations. The field of view is about 65 mm wide by 32 mm high (512 × 256 pixel). Theplane of the swirl burner orifice is located about 5 mm below the lower edge of the image. The visualized sprayexhibit a zig-zag of concentration bands inclined about 30o above normal and can be explained with the imagedprojection of a helical spiral of spray moving upstream into the combustor. The spray persists long enough to enterthe combustion zone in which the spray is eventually completely evaporated.

The spiraling mode is even more apparent in animations of the image sequences. Extracting image intensitiesat a fixed location shows a clear periodicity (figure 4). The two traces were obtained at positions to either side ofthe burner axis and show a clear 180o phase difference. This is substantiated by the negative correlation betweenthe signal at time lag zero (figure 4, bottom). The time trace data can be readily used to calculate frequency spectra.For the following examples image sequences of 1000 frames corresponding to a duration of 50 ms were analyzed.The spectra show a dominant frequency near f0 = 2.08 kHz and harmonics thereof, summarized in table 1. The

Figure 2. Schematic of shadowgraph imaging arrangement

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Table 1. Operating conditions and measured dominant frequencies fi

PDA Shadowgraphy No.1 Shadowgraphy No.2

p = 4 bar p = 4 bar p = 8 bar

T3 = 550 K T3 = 550 K T3 = 650 K

f0 2056± 1 Hz 2080± 20 Hz 2260± 20 Hz

f1 4120± 1 Hz 4160± 20 Hz 4460± 20 Hz

f2 - 6240± 20 Hz -

fundamental frequency is representative of the periodic detachment of the helical spray structure from the burnerexit into the combustion chamber. The higher harmonics are caused by the non-sinusoidal form of the time tracescaused by the cessation of spray production for the time immediately after film breakup and cluster formation.

Phase Doppler measurements were performed in parallel to these visualizations [4]. A time series of 50000samples acquired in about 0.9 seconds was used to calculate the spectrum of the axial velocity shown in figure 6.The velocity spectrum shows nearly the same dominant frequency at f0 = 2.06 kHz as the spray visualization datasuggesting a direct correlation between velocity field and spray distribution.

Following the observations of Becker & Hassa [1] it seems likely that a periodic fluctuation of radial velocityat the filmer lip from outward to inward movement is caused by a vortex convected with the mean swirl velocity,that is often found in swirling flows [6, 2]. This fluctuation in turn ruptures the liquid film by causing alternateelongations in the positive and negative radial directions. This results in a periodic release of a liquid parcel atany given location on the circumference of the filmer lip. In [2] such vortices were identified by particle imagevelocimetry (PIV) for a burner with a single swirling flow from radial swirlers and an inner body. Two vorticeswith 180o phase lag were found for the case of an inner jet and one vortex for the case without inner jet.

Conclusion and PerspectiveTo our knowledge this is the first application of high frame rate shadowgraphy to pressurized kerosene atom-

ization in combustion, made possible through the use of high power pulsed LED light source and a high speedcamera. The simplicity of the setup, its variability in terms of trigger frequency and pulse duration along with theabsence of speckle artifacts (a common drawback of lasers) are just of few attractive features of this visualizationtechnique.

The shadowgraph imaging configuration presented here was intended as a proof-of-concept and can be im-proved in several ways. The instantaneous LED emission can be increased by roughly a factor of 20 by overdrivingthe device with high currents. As demonstrated in [3] pulsed drive currents exceeding If = 200 A at τp = 2 µs arefeasible without damaging the device. Further improvements are possible with adequate interference filter usedto block the flame radiation. At present a significant part of LED light was lost due to its limited bandwidth andtransmission (< 50% at λo = 532 nm). Here it should be kept in mind that overdriving the LED results in asignificant shift of emitted light toward shorter wavelengths by up to 30 nm [3]. Image contrast can be furtherimproved by operating the camera in shuttered mode synchronized to the LED strobe flashes. At the given framerate (20 kHz) and pulse width τp = 2 µs this would result is a suppression of ambient light by a factor of 25.

The current imaging setup did not involve a diffusing screen as is common in shadowgraph setups. While thismade the setup very light sensitive it also visualized small scale temperature gradients in the turbulent flame abovethe burner due to the associated light refraction. Given the possible margins of improvement allows a scatteringscreen to be added is which case the acquired images will exhibit less temperature gradient induced Schlieren.

The temporally resolved image sequences acquired with the imaging setup allow the efficient extraction ofdominant frequencies in the ejected kerosene spray. Future work will concentrate on the coupling of unsteadyatomization with the unsteady characteristics of the gas flow. The flow field information provided by phase-lockedor time resolved PIV measurements in a plane normal to the burner axis is believed to shed light on presence ofprecessing vortices or similar cyclic features in the vicinity of the nozzle.

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Figure 3. Successive shadowgraph images of the airblast spray at pressure p = 4 bar obtained at 50 µs timeseparation (20 kHz). LED illumination was limited to 2 µs to freeze the motion of the structures

References[1] J. Becker and C. Hassa, “Experimental investigation of spatial and temporal aspects of the liquid fuel placement

in a swirl cup at elevated pressure,” in ASME Turbo Expo 2004: Power for Land, Sea and Air, Vienna, Austria,14–17 June 2004.

[2] K. Midgley, A. Spencer, and J. J. McGuirk, “Vortex breakdown in swirling fuel injector flows,” in ASME TurboExpo 2007: Power for Land, Sea and Air (GT2007), Montreal, Canada, 14–17 May 2007.

[3] C. Willert, B. Stasicki, S. Mößner, and J. Klinner, “Pulsed operation of high power light emitting diodes forimaging flow velocimetry,” Measurement Science and Technology, 2010, accepted, to appear.

[4] S. Freitag, U. Meier, J. Heinze, T. Behrendt, and C. Hassa, “Measurement of initial conditions of a kerosenespray from a generic aeroengine injector at elevated pressure,” in 23rd European Conference on Liquid Atom-ization and Spray Systems (ILASS 2010), Brno, Czech Republic, September 2010.

[5] C. Willert, S. Freitag, and C. Hassa, “High speed imaging of fuel sprays using a low-cost illuminationsource,” in 22nd European Conference on Liquid Atomization and Spray Systems (ILASS 2008), Como, Italy,September 2008. [Online]. Available: http://elib-v3.dlr.de/55164/

[6] A. Gupta, D. Lilley, and N. Syred, Swirl Flows. Tunbridge Wells, UK: Abacus Press, 1984.

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http://elib-v3.dlr.de/55164/


Figure 4. Time traces (top) and corresponding cross-correlation plot (bottom) obtained at x = 15 mm above thespray nozzle on opposite sides of the burner axis (y = ±14 mm)

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a)

b)

c)

Figure 5. Frequency spectra obtained at 3 different locations using a sequence of 1000 images: a) near the outeredge of the spray, b) at the center line, c) at an intermediate position

Figure 6. Frequency spectrum calculated from a PDA time trace of 50,000 samples obtained at x = 12 mm abovethe spray nozzle at a radial distance of z = 7 mm

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high speed shadowgraphy of a combusting air blast atomizer spray

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